This invention is a circuit for calibrating the duty cycle of a pulse width modulation system and more particularly is a closed loop feedback system for adjusting the duty cycle of a current controlled pulse width modulator to compensate for the long term variations of the system clock.
In a laser printer, a fixed frequency data clock can not be used since the data rate must be variable to make up for the tolerances in the system. For example, such things as the image size and the rotational speed of the polygon can not be kept exactly constant, and therefore the data clock rate must be varied to compensate.
A complication arises in a tri-state printer which deposits two different colorants onto the output hard copy in one pass. The colors are typically black and a highlight color of red, green or blue. In a tri-state xerographic system one of the toners is charged positively and the other is charged negatively. Then, by charging the photoreceptor with either positive or negative charges, two colorants can be applied to the photoreceptor in one pass. More specifically, the photoreceptor is originally charged to a first voltage, and then discharged either 0% for one colorant or 100% for the other colorant. In this system it is assumed that if the photoreceptor is discharged approximately 50%, it will attract neither colorant, and the result will be white, the color of the typical paper medium. Actually, the exact percent of discharge needed to get a pure white color may vary, depending on the individual system characteristics. Values of 15% to 70% are common, where 100% is full highlight color and 0% is black.
Once the correct level has been found, it is critical that it be maintained. For example, if the highlight color is red and the white level is excceeded by only a few percent, the whole page will have a pink tint. In addition, as the frequency of the clocked data changes, the On and OFF times must vary accordingly to maintain the exact duty cycle.
The amount of discharge is produced by pulse width modulation. For each pixel, the light will be on for a part of the time and off for the remainder. The actual duty cycle required for each system can be expressed as a binary number. For example, a duty cycle of 621/2%, expressed as an six bit number, would be 101000.
Setting the duty cycle of a pulse modulation system is not difficult in a system with a fixed frequency clock. The complication here is that the system clock, as explained above, is also variable. The requirement here is that the ON time must be varied as the system clock frequency varies so that the duty cycle will remain constant while the clock frequency is allowed to change.